View Compilation Error

0_0_33068800_25359.cpp: In function 'int main()':
0_0_33068800_25359.cpp:7:28: error: 'c' was not declared in this scope
     while(cin >> a >> b >> c >>d){
                            ^
0_0_33068800_25359.cpp:7:32: error: 'd' was not declared in this scope
     while(cin >> a >> b >> c >>d){
                                ^